Experimental and molecular dynamic studies of phase transformation and amorphization in silicon during high-energy ball milling

Abstract

Nano-sized and amorphous silicon (α-Si) have a wide range of properties and applications, but their atomistic structure and the process of crystal evolution are still unclear. In this study, a combined action is used to present a mechanism for the crystal-to-amorphous solid–solid phase transition in silicon during mechanical alloying (MA) at room temperature. For this purpose, the ball milling of silicon was performed and changes in crystallographic structure and amorphous phase were studied by scanning electron microscopy (SEM), X-ray diffraction patterns (XRD), and energy dispersive X-ray spectroscopy (EDX). The process of changing phase was determined by quantitative phase analysis (MAUD/Rietveld refinement). In the simulation part, a crystal structure of silicon consisting of 64 atoms was created and uniaxial and biaxial external pressures were applied randomly by BIOVIA Materials Studio. The time step and simulation time were 1 and 300 Ps, respectively. Important atomic properties including density, mean square displacement (MSD), angle length and bonds distribution, and the fraction of broken bands were measured after applying per compression. Atomic structure and changes in crystalline phase to nano and amorphous phases were studied and these results had a considerable agreement with the experimental part and well-known previous studies. The diamond cubic structure shows a remarkable toughness during the modeling and so amorphization was performed at an exponential evolution. The Results of the simulations showed that the bond angle and length distribution and MSD, were increased significantly, and in contrast, density was decreased to 1.254 g/cm3. The percentage of the broken Si-Si bonds was 58.12 % and a new definition of Si amorphization based on the fraction of covalent bonds was proposed. The amorphization started from the surface of Si and spread by the partial distribution toward the crystal structure. These findings show that the proposed mechanism based on the fraction of breaking bonds and pressure-induced amorphization could be used for the accurate simulation of the solid–solid phase transformation.